The present invention relates to an optical output control circuit which is capable of being kept intensity of a light signal to required value.
Description of the Prior Art
When it causes intensity of a light signal which is sent out to an optical fiber to be set to constant value, an optical output control circuit for keeping intensity of the light signal to constant value is employed. Generally, in order to set intensity of the light signal to the constant value, an optical output control circuit which can keep intensity of the signal light to the constant value is employed. Further, when it causes the inputted light signal to be supplied to an optical module which is limited in reception level of the light signal, intensity of the input light is necessary to be lowered. In such the case, the optical output control circuit is employed.
FIG. 1 is a block diagram showing a constitution of a conventional optical output control circuit. The optical output control circuit 200 shown in FIG. 1 sets intensity of the light signal inputted from the optical fiber 201 to the prescribed value. The optical output control circuit outputs the light signal with the prescribed value to the optical fiber 207. In the optical output control circuit 200, the inputted light signal is inputted to an optical branching part 203 while passing through a variable light attenuator 202. The optical branching part 203 inputs the light signal to the optical fiber 207 and a light-electricity conversion circuit 206 while branching the inputted light signal.
The light-electricity conversion circuit 206 converts the light signal as the inputted monitor light into an analog electric signal. Intensity of the monitor light is correlated with amplitude of the analog electric signal. Further, the analog electric signal is inputted to an attenuator control circuit 205. The attenuator control circuit 205 prepares control signal for specifying attenuation quantity of the variable light attenuator 202 in accordance with amplitude level of the analog electric signal to output to the variable light attenuator 202. The variable light attenuator 202 sets attenuation quantity in accordance with the control signal.
The attenuator control circuit 205 outputs a control signal instructing that it causes the attenuation quantity to be increased when the amplitude level of the analog electric signal is larger than a prescribed value, while when the amplitude level of the analog electric signal is smaller than the prescribed value, the attenuator control circuit 205 outputs the control signal instructing that it causes the attenuation quantity to be reduced. According to such control, the intensity of the light signal outputted to the optical fiber 207 is kept to prescribed value. Further, such the optical output control circuit is disclosed in the Japanese Patent Application Laid-Open No. HEI 4-150324 or the Japanese Patent Application Laid-Open No. HEI 10-336114.
The conventional optical output control circuit can merely implement an optical output constant control for keeping intensity of the light signal to the constant value. The conventional optical output control circuit can not implement more wider control than the above described optical output constant control. For instance, although the optical output control circuit it self prepares a monitor signal concerning intensity of the light signal, when there is a demand that it causes intensity of output light to be monitored in external part, it is incapable of replying to the demand that it causes intensity of output light to be monitored at the external part simply by using the conventional optical output control circuit. For instance, generally a photodiode (PD) is employed as the light-electricity conversion circuit 206 which generates a monitor signal. However, input-output characteristic of the photodiode (PD) is not linear, consequently, even though it causes output of PD to be observed at the external part, it is incapable of recognizing intensity of the light signal immediately from the monitor signal. Further, there exists dispersion in the characteristic of the PD, thus it is not necessarily that intensity of the light signal outputted from respective optical output control circuits is identical with each other, even though the monitor signal from the plurality of optical output control circuits indicates the same analog electric level.
Furthermore, the Japanese Patent Application Laid-Open No. HEI 8-37499 discloses control circuit which keeps intensity of the light signal into a prescribed value according to digital processing by CPU. Conversion table is included in the control circuit. However, the control circuit obtains attenuation quantity from the conversion table in accordance with the monitor level on that occasion, further the control circuit confirms the output light intensity in accordance with obtained attenuation quantity. Consequently, in order to obtain required output light intensity, it should execute frequently that it causes light intensity to be confirmed while changing attenuation quantity very little by very little. Moreover, the control circuit is not considered concerning signal monitoring and so forth from the external part.